A while ago we had a custom snowmobile exhaust formed by BTD Manufacturing, though they supplied just raw CNC bent tubing created from autocad files, we welded it together.
I'd probably get a stock katana exhaust for your engine, and try to modify it to fit. At the very least you can cut it up to use the flanges for the headers and work from there, at least give a shop something to start with.
If it's not even close or you don't have the resources to do that, start looking up fabrication/metalworking/welding shops in your area. You might find a bike customs shop that's willing to create an exhaust, but you can be sure it won't be cheap. There's a lot of work in measuring, manufacturing, and fitting an exhaust system from scratch, which is why if at all possible I'd try to modify a stock one.
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